Phased array antennas are composed of a large number of individual radiating elements that are separately exited. When circular polarization is used, it is desirable to increase the capacity of the system by providing two separate and isolated antenna beams, one with left hand circular polarization (LHCP) and the other with right hand circular polarization (RHCP). Since much of the basic research in the area of phased array antennas has been done with waveguides, this type of element becomes desirable to simplify the design of radomes or matching layers which can optimize axial ratio and control element impedance variation as the array is steered across its scan angle range. Waveguide elements can also be preferred because they provide wider frequency bandwidth and better isolation between elements than dipole or patch antennas, for example.
In a phased array antenna, each radiating element converts the RHCP and LHCP waves into the respective linearly polarized orthogonal modes which, in general, are perpendicular to each other and of approximately equal power. In addition, the dominant modes are processed so as to have a 90.degree. phase difference in time. One approach to such processing is to provide two probes with perpendicular alignment to each other in space, and a quadrature hybrid coupler external to the waveguide to provide the 90.degree. phase difference. For example, Howard U.S. Pat. No. 5,043,683, shows two probes in the form of linear antenna segments projecting inward from the side of a circular waveguide at an angle of 90.degree. relative to each other. A quadrature hybrid coupler external to the waveguide (see, for example, FIG. 2 of U.S. Pat. No. 5,043,683) processes the signals from the respective probes to achieve the desired 90.degree. phase difference.
A second approach is to use two mutually perpendicular probes and a differential phase delay polarizer internal to the waveguide. The polarizer is oriented at 45.degree. to both of the probes. A similar approach is used in the construction of Withers U.S. Pat. No. 4,707,702, where a single probe projects radially through the side of a circular waveguide containing a polarizer inclined at an angle of 45.degree. to the probe and located specifically to result in two orthogonal waves, one delayed by 90.degree. more than the other.
Problems with the known approaches include: precise positioning is required for multiple components; bulky elements may be required external to the waveguides; elements project from the sides of the waveguides and thereby affect the close packing required for an efficient phased array antenna; as well as impedance matching and isolation characteristics.